Field of the Invention
The present invention pertains to solid polymer electrolyte fuel cells comprising selectively conducting anodes which improve durability, and to methods and constructions for obtaining desirable performance and tolerance to voltage reversal.
Description of the Related Art
Sustained research and development effort continues on fuel cells because of the energy efficiency and environmental benefits they can potentially provide. Solid polymer electrolyte fuel cells are particularly suitable for consideration as power supplies in traction applications, e.g. automotive. However, improving the durability of such cells to repeated exposure to startup and shutdown remains a challenge for automotive applications in particular.
Unacceptably high degradation rates in performance can occur in solid polymer electrolyte fuel cells subjected to repeated startup and shutdown cycles. The degradation can be further exacerbated when using low catalyst loadings in the electrodes for cost saving purposes. Often, there is a trade-off between durability and cost in the fuel cell. During the startup and shut-down of fuel cell systems, corrosion enhancing events can occur. In particular, air can be present at the anode at such times (either deliberately or as a result of leakage) and the transition between air and fuel in the anode is known to cause temporary high potentials at the cathode, thereby resulting in carbon corrosion and platinum catalyst dissolution. Such temporary high cathode potentials can lead to significant performance degradation over time. It has been observed that the lower the catalyst loading, the faster the performance degradation. The industry therefore needs to find means to address the performance degradation.
A number of approaches for solving the degradation problem arising during startup and shutdown have been suggested in the art. For example, the problem has been addressed by employing higher catalyst loadings, valves around the stack to prevent air ingress into the anode during storage, and using carefully engineered shutdown strategies. Some suggested systems incorporate an inert nitrogen purge and nitrogen/oxygen purges to avoid damaging gas combinations being present during these transitions. See for example U.S. Pat. No. 5,013,617 and U.S. Pat. No. 5,045,414.
Some other concepts involve fuel cell stack startup strategies involving fast flows to minimize potential spikes. For example, U.S. Pat. No. 6,858,336 and U.S. Pat. No. 6,887,599 disclose disconnecting a fuel cell system from its primary load and rapidly purging the anode with air on shutdown and with hydrogen gas on startup respectively in order to reduce the degradation that can otherwise occur. While this can eliminate the need to purge with an inert gas, the methods disclosed still involve additional steps in shutdown and startup that could potentially cause complications. Shutdown and startup can thus require additional time and extra hardware is needed in order to conduct these procedures.
Recently, in PCT patent application serial number WO2011/076396 by the same applicant, which is hereby incorporated by reference in its entirety, it was disclosed that the degradation of a solid polymer fuel cell during startup and shutdown can be reduced by incorporating a suitable selectively conducting component in electrical series with the anode components in the fuel cell. The component is characterized by a low electrical resistance in the presence of hydrogen or fuel and a high resistance in the presence of air (e.g. more than 100 times lower in the presence of hydrogen than in the presence of air).
It was noted in WO2011/076396 however that the presence of a selectively conducting component or layer could potentially lead to a loss in cell performance (due to an increase in internal resistance) and also could lower the tolerance of the fuel cell to voltage reversals. Still, judicious choices of components (e.g. such as those illustrated in the Examples) can be effective for improving durability with only a minimal, acceptable effect on performance. And a remedy for a lowering in voltage reversal tolerance was suggested. Instead of extending the layer of selectively conducting material over the entire active surface of the anode, some regions could be provided where the layer was absent to allow for dissipation of reversal currents and/or provide a sacrificial area in the event of cell reversal. Further, it was mentioned that it may be advantageous to keep the selectively conducting layer separate from the anode catalyst. A carbon sublayer may for instance be incorporated between the two for this purpose. While this can also provide a potential solution for voltage reversal tolerance, it can adversely affect performance.
It was thus found to be difficult to simultaneously achieve commercially preferred voltage reversal tolerance and commercially preferred performance as well as startup/shutdown durability. In later patent application US2014/0030625 however, an improved approach was disclosed to address the problem of lower voltage reversal tolerance when using a selectively conducting anode component in such cells. Fuel cells exhibiting acceptable behaviour in every regard could be obtained by incorporating a carbon sublayer in contact with the side of the anode opposite the solid polymer electrolyte, and appropriately selecting the selectively conducting material and carbon sublayer such that the fuel cell voltage is greater than about 0.5 V when operating at 1.5 A/cm2. However, while acceptable, cells incorporating such sublayers did not perform quite as well as cells without such sublayers.
There thus remains a desire for improvement in fuel cells comprising selectively conducting anodes, and specifically for improvement in performance and tolerance to voltage reversal. The present invention fulfills this and other needs.